Phosphor dielectric storage target for cathode ray tube



Dec. 20, 1966 c. B. GIBSON, JR

PIIOSPHOR DIELECTRIC STORAGE TARGET FOR CATHOHE RAY TUBE Filed Aug.

STORAGE TUBE CHARLES E GIBSON, JR

//V I E N TOR BUG/(HORN, BLORE, KLAROU/S T 8 SPAR/(MAN A T 7' OR/VE Y5 United States Patent 3,293,474 PHUSPHOR DIELECTRIC STORAGE TARGET FGR fiATHODE RAY TUBE Charles B. Gibson, Jr., Portland, 0reg., assignor to Tektronix, llnc., Beaverton, Greg, a corporation of Oregon Filed Aug. 1, 1963, Ser. No. 299,422 7 Claims. (Cl. 313-68) The subject matter of the present invention is related generally to electron image storage systems, and in particular to .a storage target for a cathode ray tube which stores an electron image formed on such target for an indefinite controllable time and produces a light image and/or an electrical signal corresponding to such electron image. The storage target includes :a mesh-like electrode of conducting matenial coated on one side of a support plate of insulating material, and a storage dielectric layer coated on such one side of the support plate in the form of spaced dielectric are-as which are separated by the elements of the mesh electrode. The dielectric areas may be made of phosphor material if direct viewing is required and are separated from each other so that they store a bistable charge image formed thereon for an indefinite controllable time as well as emitting a light image corresponding to such charge image, in which case the support plate is made of light transparent material. The present invention also includes photographic methods of manufacture of storage targets employing a mesh electrode, such as that described above, in order to obtain a fine mesh pattern of conducting material for such electrode to increase image resolution.

A storage target made in accordance with the present invention is especially useful when employed in a bistable direct viewing type of storage tube which forms part of a cathode ray oscilloscope to store the wave forms of transient electrical input signals for extended examination. However, such a storage tube can also be utilized in the manner of any conventional storage tube, such as in radar apparatus, sonar apparatus or electronic computers. The storage target of the present invention has several advantages over conventional storage target-s including an increased maximum Writing speed due to the lower capacitance of the storage target so that such target is capable of storing the electron images of high frequency information. This reduced capacitance is the result of the use of a mesh electrode in the storage target, rather than a continuous electrode beneath the storage dielectric of such storage target. In addition, a direct viewing storage target in accordance with the present invention produces a light image of excellent contoast.

The photographic methods of manufacture of the storage target of the present invention are simple and economical and even so, produce targets having image resolution comparable to conventional storage tubes employing wire mesh electrodes in their storage targets, such as that shown by F. H. Harris in US. Patent No. 2,839,679, issued June 17, 1958. The storage target structure of the present invention is simple, hugged and reliable since both the mesh electrode and the storage dielectric layer are applied as coatings onto :a common support plate of insulating material. Thus, for large diameter storage tubes there are no problems of increasing the thickness of the elements of the mesh electrode for greater strength and of supporting the electrode as a planar structure in the present storage target, as is the case with conventional targets employing a wire mesh electrode. Another advantage of the present target structure is that it enables the spaced, storage dielectric areas of such target to be substantially uniform so that there is ice less noise or shading due to field curvature, in the electrical readout signal produced by scanning the target with a reading beam.

In addition to the above advantages, the present storage target has a substantially uniform electrical potential distribution over the rear surface of the target, even when a charge image is stored thereon, because the conductive elements forming the mesh electrode are exposed between the storage dielectric areas of such target. Since the elements of the mesh electrode are the same potential, the changes in potential of the dielectric areas have little effect on the over-all potential distribution at the rear 7 surface of the target so that such potential distribution remains substantially uniform. Previous targets employing :an electrode beneath a continuous layer of storage dielectric have a nonuniform potential distribution on the rear surface of the target due to the different charges stored on adjacent areas of the dielectric layer. This nonuniform potential distribution effects the transmission of low velocity flood electrons to the storage target, which are employed to hold the charge image produced thereon by the writing beam, and produces an effect similar to the coplanar grid effect which distorts the charge image. In order to compensate for this nonuniform potential distribution, a collimating electrode is provided in previous tubes in the form of a conductive wall coating on the tube envelope and such electrode is connected to a source of DC. voltage in order to distribute the flood electrons uniformly over the surface of the storage target. This is eliminated by the structure of the storage target of the present invention.

It is therefore one object of the present invention to provide an improved apparatus for storing electron images.

Another object of the invention is to provide an improved storage target for storing an electron charge image thereon which has a low target capacitance and an increased maximum writing speed.

A further object of the present invention is to provide an improved direct viewing storage target in which a phosphor material is employed as the storage dielectric for storing an electron image formed thereon and for produc ing a light image corresponding to the electron image of excellent brightness, increased contrast and high resolution.

An additional object of the invention is to provide an improved cathode ray storage tube in which a storage target having a simple structure which is rugged and reliable is employed thus enabling a storage tube of large size to be produced without decreasing the resolution of the image stored on such target.

Still another object of the invention is to provide an improved storage target in which a mesh electrode in the form of a conductive coating on a support member of insulating material and a storage dielectric layer in the form of a plurality of spaced dielectric areas coated on such support member within the apertures of such mesh electrode result in a storage target capable of use at high writing speeds of an electron beam.

A still further object of the present invention is to provide an improved method of manufacture of a storage target in which a photographic process is employed to produce a mesh electrode as a uniform fine mesh coating of conducting material on an insulating support and a storage dielectric layer including a plurality of spaced storage areas contained within the apertures of such mesh electrode.

Other objects and advantages of the present invention will be apparent from the following detailed description of preferred embodiments thereof and from the attached drawings of which:

FIG. 1 is a diagrammatic view of one embodiment of a storage tube in accordance with the present invention and associated electrical circuity;

FIG. 2 is a partial horizontal section view taken along the line 22 of FIG. 1 showing on an enlarged scale one embodiment of a storage target in accordance with the present invention.

FIG. 3 is a view taken along the line 33 of FIG. 2 showing the rear surface of the storage target on an enlarged scale; and

FIG. 4 is a partial sectional view of another embodiment of the storage target.

A direct viewing, bistable storage tube having a storage target 12 made in accordance with the present invention is shown in FIG. 1. This storage tube may have a single electron gun including a cathode 14-, a control grid 16, a focusing anode structure 18 as well as a pair of horizontal deflection plates 20 and a pair of vertical deflection plates 22. This single electron gun may be employed to produce either a writing beam or a reading beam of electrons by changing the positions of each of three ganged switches 24, 26 and 28 connected, respectively, to the control grid 16, horizontal deflection plates 20 and vertical deflection plates 22 between a WRITE position and the READ position, in a manner hereafter described. However, it should be understood that a pair of separate electron guns may be employed to form the writing beam and the reading beam. The writing beam forms an electron charge image on the storage target 12 by deflection of the writing beam across such storage target in accordance with an input signal applied to the vertical deflection plates 12. The reading beam is employed to produce an electrical readout signal on the storage target of the storage tube by scanning the charge image stored on the storage target 12, for example, in accordance with a conventional television raster pattern.

One or more flood guns 30 may be provided within the envelope of the storage tube 10 in order to bombard the surface of the storage target 12 substantially uniformly with low velocity flood electrons in order to maintain or hold the charge image produced on such storage target by the writing beam after such writing beam no longer bombards such target. The storage target 12 includes a mesh electrode 32 shown in FIGS. 2 and 3 which is connected to a DC. target voltage across load resistor 34, and a storage dielectric 36 in the form of a plurality of spaced dots of rectangular, circular or other suitable configurations. When the target voltage applied to mesh electrode 32 is within the stable range of target voltages over which the dielectric layer 36 of the storage target will store a charge image for an indefinite controllable time, the writing beam of high velocity electrons produces by secondary emission a charge image on the dielectric layer 36 which is more positive than the areas not struck by the beam. The potential of the written charge image is above a critical voltage corresponding to the first crossover point on the secondary emission curve of such dielectric, while the remaining unwritten areas of such dielectric layer have a potential below such critical voltage. The flood electrons bombarding the storage target drive the potential of the written areas of the dielectric layer 36 to a high voltage stable state corresponding to the potential on the mesh electrode, and drive the potential of the unwritten areas to a low voltage stable state corresponding to the voltage applied to the cathode of the flood guns 30. This bistable storage operation has been previously described in copending U.S. patent application Serial No. 180,457 by Robert Anderson, filed March 19, 1962, entitled Electron Discharge Display Device.

The load resistor 34 is connected to a DC. voltage source of +500 volts through a variable bias resistor 38 whose setting controls the target voltage applied to the mesh electrode 32. In order to erase the charge image stored on target 12 the variable resistor 38 is decreased in value until the voltage applied through the mesh electrode exceeds the fade positive voltage of the dielectric layer 36 so that the flood electrons cause the potential of the dielectric layer to become uniformly written positive. Next the resistance of the variable resistor 38 is increased until the target voltage applied to the mesh electrode 32 is below the retention threshold voltage for the dielectric layer 36 below which the storage target will not store a charge image. Then the voltage applied to the mesh electrode is raised above the retention threshold volt-age to a voltage within the stable range of target voltages over which the dielectric layer will store a charge image and the target is ready to receive another charge image.

During the writing operation of the storage tube of FIG. 1 the control grid 16 is connected by a switch 24 to a source of DC. voltage of 3,025 volts which is slightly negative with respect to the DC. voltage of 3,000 volts applied to cathode 14. The horizontal deflection plates 20 are connected by a switch 26 to a horizontal sweep generator 40, while the vertical deflection plates 22 are connected by a switch .28 to a vertical amplifier 42, which may be of the conventional type employed in cathode ray oscillosoopes. The input signal whose wave form is to be stored on the storage target 12, is applied to the input terminal 44 of the vertical amplifier 42.

The dielectric layer 26 of the storage target 26 may be made of phosphor material including conventional phosphors, such as P-1 type phosphor, and even photoconductive phosphors, so that the dielectric layer produces a light image corresponding to the charge image stored thereon when the storage tube is of a direct viewing type. In this case it is not necessary to provide the storage tube with an electrical readout circuit since the wave form of the input signal can be observed directly through the face plate of the storage tube. Photoconductive phosphors may be used as the storage dielectric because such dielectric is in the form of a plurality of separate spaced areas or dots which may be insulated from the target electrode, as shown in FIG. 4, to prevent the charge image stored on such dielectric from spreading due to photocond-uctivity. A collimating electrode 4-5 may be provided as a wall coating of conductive material on the interior surface of the funnel portion of the envelope adjacent the storage target 12. This collimating electrode may be connected to a DC. voltage of +50 volts to focus the flood electrons onto the storage target and to prevent distortion of the stored image due to the coplanar grid effect discussed previously by which the positive target areas attract some of the flood electrons away from the adjacent negative areas.

However, if the dielectric layer 36 is not of phosphor, but is of another secondary emissive material, the storage tube must be provided with a readout circuit to produce an electrical readout signal corresponding to the charge image stored on the storage target. This may be accomplished by connecting the control grid 16 by switch 24 to a source of DC. voltage of -3,050 volts in order to reduce the current density of the electron beam transmitted from the cathode 14 to target 12 in order to prevent such reading beam from producing a stored image on the target. In addition, the horizontal deflection plates 20 and the vertical deflection plates 22 may be connected by switches 26 and 28, respectively, to a raster signal generator 46. The raster signal generator applies conventional saw tooth signals of different frequency to the horizontal plates and to the vertical plates in order to produce a conventional television raster scanning pattern for the reading beam. This raster pattern can be controlled to cover all, or only a portion of the storage target 12 in order to magnify a portion of the image stored thereon. This image magnification operation can be performed automatically by adjusting the raster signal generator so that the vertical raster signal applied to the deflection plates 22 of the storage tube runs between two voltage limits which correspond to voltages on opposite sides of the wave form portion sought to be magnified. The electrical readout signal produced on the mesh electrode 32 is transmitted through a coupling capacitor 48, a low impedance preamplifier 50 and a high gain amplifier 52 to the Z-axis input of a remotely positioned television monitor tube 54 or other recording device. The horizontal and vertical deflection plates of the monitor tube 54- are also connected to the raster signal generator 46 so that the monitor tube displays the entire wave form image stored on the storage target 12 of the storage tube, or only a magnified portion of such wave form. Of course, it may be desirable to employ such a television monitor tube and electrical readout circuit even when the storage dielectric layer 36 of the target 12 is phosphor material in order to enable the remote observation of the stored wave form or to enable magnification of a portion thereof as indicated.

As shown in FIGS. 2 and 3, one embodiment of the storage target 12 of the present invention employs the mesh electrode 32 as a coating of conductive material on the surface of one side of a light transparent support plate 56 of insulating material which may be the glass face plate portion of a cathode ray tube envelope. The envelope may include a funnel portion 58 of ceramic material which is sealed to the face plate 56 by a glass frit seal 60. The mesh electnode 32 may be formed of a light opaque conductive material, such as aluminum or silver, or a light transparent conductive material, such as tin oxide. When the mesh electrode is made of tin oxide, it may extend through the seal 60 to the exterior of the envelope in order to provide an electrical lead portion 61 connected to the mesh electrode within the envelope. The dielectric layer 36 of the storage target may be provided as a plurality of spaced storage areas 62 of phosphor or other dielectric material which are coated on the inner surface of the face plate 56 within the apertures of the mesh electrode 32 so that such storage areas are separated from each other by the elements of such mesh electrode. The storage dielectric areas 62 are disposed laterally of the mesh electrode elements directly in contact with the face plate 56, and are not between the writing beam and a continuous conductive layer on such face plate as in some prior target-s, so that the capacitance formed by such dielectric areas which must be charged by the Writing beam bombarding the storage target, is materially reduced. The dielectric areas 62 may be rectangular, circular or of any other suitable configuration and are all of approximately the same thickness to provide a substantially uniform low capacitance over the surface of the storage target. Also the outer edges on the rear surface of these areas may be formed with rounded corners rather than square corners in order to produce this uniform capacitance because of the increased electrostatic fields produced at sharp corners.

A graticule scale 64 in the form of a plurality of scribed lines may be provided on the inner surface of the face plate 56 beneath the storage target 12 when the dielectric layer 36 is of phosphor in order to provide an internal .graticule for examining the light image produced on the storage target. This internal graticule maybe edged lighted by projecting light through the surrounding edge of the face plate 56. in addition, the graticule scale 64 may also be provided by deposits of glass frit printed on the inner surface of such face plate to produce the graticule lines.

There are several photographic methods of making the storage target structure of FIGS. 2 and 3. One of these methods includes the steps of (1) coating one side of the face plate 56 with conductive material such as tin oxide, aluminum or silver; (2) applying a layer of photosensitive material over this conductive coating; (3) exposing the photosensitive layer to the light image of a grid or rnesh produced by transmitting light through a photographic film negative, containing a positive image of such mesh, placed in contact with such photosensitive layer;

(4) developing the latent image produced on such photosensitive layer by such light image; (5) etching the coated face plate to remove the unexposed portions of the photosensitive layer and the underlying areas of the conductive coating by employing a solution of hydrochloric acid and zinc metal powder; (6) removing the exposed photosensitive material remaining over the mesh electrode coating produced by the preceding steps; (7) applying a layer to the face plate 56 over the mesh electrode containing a mixture of phosphor or other dielectric material and photosensitive material. For example, such a layer may include 10 grams of phosphor for milliliters of photosensitive solution, the photosensitive solution containing 100 grams of polyvinyl alcohol, 1000 milliliters of water, 1.0 milliliter is isopropanol and 20 grams of ammonium dichromate activator; (8) exposing the photosensitive dielectric layer to light through the mesh electrode coating when such coating is light opaque or through a mesh shaped mask in registration with such conductive coating when the conductive coating is light transparent; (9) washing the surface of the coated face plate with water to remove unexposed regions of the photosensitive dielectric layer; and (10) heating the resulting structure to 400 C. for thirty minutes in order to remove the photosensitive material from the dielectric layer including organic binders and other impurities.

A second photographic method which may be employed to produce the storage target 12 of FIGS. 2 and 3 includes the steps of (l) coating one side of the glass face plate 56 with a photosensitive solution containing a mixture of polyvinyl alcohol photosensitive solution, such as that of step 7 of the first method, and a powder, such as silicate, P1 phosphor or metal oxide, which is insoluble in such photosensitive solution; (2) exposing the photosensitive layer to the light image of a uniform mesh by transmitting the light through a suitable photographic negative; (3) developing the negative mesh image by washing the coated plate in water to remove the unexposed portions of photosensitive material; (4) coating the plate uniformly with a layer of conducting material such as tin oxide, aluminum or silver by evaporation so that such conductive layer contacts the surface of the face plate in a positive mesh image and also covers the exposed photosensitive material; (5) heat the coated face plate at 400 C. for thirty minutes to remove the polyvinyl alcohol hotosensitive material; (6) remove the spaced areas of powder material remaining on the plate and those portions of the coducting layer overlying such powder areas by the use of air pressure or abrasive material, and repeat 7 to 10 of the first method. It should be noted that steps 4 and 6 of the second method can be eliminated if the powder material employed in step 1 is capable of forming a conductive coating which adheres to the face plate after heating, and that in this case the exposure step must form a positive mesh image.

A third method which can be employed to produce the storage target 12 of FIGS. 2 and 3 includes the steps of (1) exposing a stripping type of photographic film to a negative light image of the mesh pattern; (2) developing the latent image on the stripping film and transferring such film to a stop bath; (3) etching the film to removing exposed negative image areas of the silver halide emulsion layers from the acetate base by applying a solution of cupric nitrate, hydrogen peroxide and acetic acid; (4) exposing the film uniformly to produce a positive image on the remaining portion of the emulsion layer; (5) developing such positive image and fixing the film in hypo; (6) soaking the film in warm water to loosen the remaining portion of the emulsion layer from the acetate base; (7) placing the outer surface of the emulsion layer into contact with the glass face plate; (8) removing excess water from the film and face plate to insure good adhesion between the emulsion layer and the face plate and drying such film; (9) removing the acetate base of the film from the emulsion layer by dissolving such base in acetone to leave the emulsion layer on the surface of the face plate in the form of a positive mesh image; (10) heating the coated face plate to 380 C. to remove any residual acetate; (11) performing steps 7 to 10 of the first method; and (12) heating the coated plate at 450 C. briefly to reduce the silver halide image to a conductive silver mesh electrode. It should be noted that the silver halide emulsion can be applied directly to the glass face plate as a liquid coating, in which case steps 6 to 10 of the third method may be eliminated.

FIG. 4 shows an alternative embodiment of the storage target 12 of the present invention. In this embodiment the glass face 56 is provided with a plurality of shallow cavities 66 on the inner surface thereof. The cavities 66 are separated by a plurality of intersecting ridges 68 to form a mesh pattern on the inner surface of the face plate. This may be accomplished by etching the glass face plate through a masking coating of photosensitive material whiich has been exposed to the light image of the mesh and developed by removing the unexposed portions of the photosensitive material. The mesh pattern may also be formed by depositing a plurality of intersecting lines of glass frit material onto the surface of a flat glass face plate and fusing such glass frit lines to produce the ridges 68. The mesh electrode 32 is then provided as a coating of silver, aluminum or other conductive material on the tops of the ridges 68, while the spaced dielectric areas 62 forming the dielectric layer 36' are deposited within the cavities 66 so that they are spaced from each other by the ridges 68 and by the elements of the mesh electrode 32.

The storage target 12' of FIG. 4 may be formed by a fourth photographic method including the steps of (1) coating one side of the face plate 56' uniformly with a layer of conductive material such as tin oxide, aluminum or silver; (2) applying a layer of photosensitive material over the conductive coating; (3) exposing the photosensitive layer to a light image of a mesh; (4) developing and fixing the positive mesh image produced on the photosensitive layer; (5) etching the conductive layer and the glass face plate with a solution of hydrogen fluoride through the unexposed areas of the photosensitive layer which forms a mask for the etching liquid; (6) removing the remaining portion of the photosensitive layer by employing a suitable stripping solution; (7) filling the cavities in the glass face plate with phosphor material by using a doctors blade; and (8) heating the face plate to 400 C. for 30 minutes to bake out any impurities in the phosphor material.

All of the methods for making the storage target described above have at least one photographic exposure step in which a photosensitive layer is exposed to the light of a mesh pattern in order to produce the mesh electrodes 32 and 32. This enables such mesh electrode to be provided with a plurality of substantially uniform, small apertures and with a plurality of thin mesh elements so that a large number of small phosphor areas 62' may be provided within the apertures of the mesh electrode. The result is a fine mesh storage target which enables the storage of electron images and the production of light images of extremely high resolution which has not been possible with conventional mesh storage targets formed by a plurality of interwoven wires.

It will be obvious to those having ordinary skill in the art that many changes may be made in the details of the above-described preferred embodiments of the present invention without departing from the spirit of the invention. Therefore, the scope of the present invention should only be determined by the following claims.

I claim:

1. A storage target for storing an electron image, comprising:

a nonconducting support member of electrical insulative material;

a coating of conductive material in contact with one 8 side of said support member having a plurality of separated apertures which extend through said coating to said support member; and

a storage dielectric layer of secondary emissive phosphor material in contact with said one side of said support member in the form of a plurality of spaced dielectric storage areas contained in the apertures of the conductive coating, said dielectric areas being separated from each other by the elements of said conductive coating to enable the bistable storage of electron images formed thereon.

2. A direct viewing storage target for storing an electron image and producing a light image corresponding to such electron image, comprising:

a light transparent support member of electrical insulative material;

a coating of conductive material in contact with one side of said support member in the form of a mesh having a plurality of separated apertures which extend through said coating to said support member; and

a storage dielectric layer of secondary emissive phosphor material in contact with said one side of said support member in the form of a plurality of spaced phosphor storage areas contained in the aperture of said conductive coating, said phosphor areas being separated from each other by the elements of said mesh to enable the bistable storage of electron images formed thereon and to produce light images corresponding to said electron images.

3. A direct viewing bistable storage target for storing an electron image and producing a light image corre sponding to such electron image, comprising:

a light transparent support plate of glass material;

a light transparent mesh coating of conductive material in contact with one side of said support plate having a plurality of separated uniform apertures which extend through said coating to said support plate; and

a discontinuous storage dielectri layer of secondary emissive phosphor material in contact with said one side of said support plate in the form of a plurality of spaced, substantially uniform phosphor storage areas contained in the apertures of said mesh coating, said phosphor areas being separated from each other by the elements of said mesh coating to enable the bistable storage of electron images formed thereon and extending into contact with said support plate so that the light images produced by said phosphor areas corresponding to said electron images are transmitted through said support plate.

4. A direct viewing storage target for storing an elec tron image and producing a light image corresponding to such electron image, comprising:

a light transparent support plate of electrical insulative material having a plurality of spaced depressions on the surface of one side of said plate;

a mesh electrode of conductive material coated on said one side of said support plate between said depressions and having a plurality of separated apertures which extend through said coating into registration with the depressions on said support plate; and

a discontinuous dielectric layer of secondary emissive phosphor material on said one side of said support plate in the form of a plurality of spaced phosphor storage areas contained in the depressions of said support plate, said phosphor areas being separated from each other by the elements of said mesh electrode to enable the storage of electron images formed thereon and extending into contact with said support plate so that the light images produced by said phosphor areas corresponding to said electron images are transmitted through said support plate, said elements or" said mesh electrode being positioned adjacent the top surfaces of said phosphor areas to increase the writing rate of said storage target.

5. A direct viewing storage target for storing an electron image and producing a light image corresponding to such electron image, comprising:

a light transparent support plate of electrical insulative material having a plurality of intersecting ridges on the surface of one side of said plate which form a plurality of spaced containers separated by said ridges,

a mesh electrode of conductive material coated on the tops of said ridges of said support plate between said containers and having a plurality of separated apertures which extend through said coating into registration with the containers on said support plate; and discontinuous dielectri layer of secondary emissive phosphor material on said one side of said support plate in the form of a plurality of spaced phosphor storage areas contained in the depressions of said support plate, said phosphor areas being separated from each other by the elements of said mesh electrode to enable the storage of electron images formed thereon and extending into contact with said support plate so that the light images produced by said phosphor areas corresponding to said electron images are transmitted through said support plate.

A cathode ray storage tube, comprisinng:

an evacuated envelope;

writing means mounted within said envelope for bombarding said storage target with a writing beam of high velocity electrons and for deflecting said writing beam across said storage target to produce an electron image on said dielectric layer and to cause said dielectric layer to emit a light image corresponding to said electron image; and

holding means mounted within said envelope for bombarding said storage target substantially uniformly with low velocity electrons in order to cause said electron image to be stored bistably for an indefinite controllable time on said dielectric layer,

7. An electron storage tube, comprisinng:

an evacuated envelope;

a storage target mounted within said envelope, including a mesh electrode coating of conductive material in contact with one side of a light transparent glass face plate forming part of said envelope, and a storage dielectric layer of phosphor material applied in contact with said one side of said face plate as a plurality of spaced storage areas which are contained within the apertures of said mesh electrode and are separated by the elements of said mesh electrode, said mesh electrode being electrically connected to the exterior of said envelope;

writing means mounted within said envelope for bombarding said storage target with a writing beam of high velocity electrons and for deflecting said writing beam across said storage target to produce an electron image on said dielectric layer and to cause said dielectric layer to emit a light image corresponding to said electron image;

holding means mounted Within said envelope for bombarding said storage target substantially uniformly with low velocity electrons in order to cause said electron image to be stored bistably for an indefinite controllable time on said dielectric layer; and

reading means mounted within said envelope for scanning said storage target with a reading beam of electrons in order to produce an electrical readout signal on said mesh electrode which corresponds to said electron image stored on said dielectric layer.

References Cited by the Examiner UNITED STATES PATENTS 2,594,740 4/1952 De Forest et al 313-68 X 2,683,832 7/1954 Edwards et al 31368 X 2,788,466 4/1957 Hansen 313--68 X 2,813,223 11/1957 Kalfaian 31368 X 2,839,679 6/1958 Harris 328-124 2,905,849 9/1959 Kazan 31392 3,087,086 4/1963 Turner 313-92 JAMES W. LAWRENCE, Primary Examiner.

R. SEGAL, Assislant Examiner. 

1. A STORAGE TARGET FOR STORING AN ELECTRON IMAGE, COMPRISING: A NONCONDUCTING SUPPORT MEMBER OF ELECTRICAL INSULATIVE MATERIAL; A COATING OF CONDUCTIVE MATERIAL IN CONTACT WITH ONE SIDE OF SAID SUPPORT MEMBER HAVING A PLURALITY OF SEPARATED APERTURES WHICH EXTEND THROUGH SAOD COATING TO SAID SUPPORT MEMBER; AND A STORAGE DIELECTRIC LAYER OF SECONDARY EMISSIVE PHOSPHOR MATERIAL IN CONTACT WITH SAID ONE SIDE OF SAID SUPPORT MEMBER IN THE FORM OF A PLURALITY OF SPACED DIELECTRIC STORAGE AREAS CONTAINED IN THE APERTURES OF THE CONDUCTIVE COATING, SAID DIELECTRIC AREAS BEING SEPARATED FROM EACH OTHER BY THE ELEMENTS OF SAID CONDUCTIVE COATING TO ENABLE THE BISTABLE STORAGE OF ELECTRON IMAGES FORMED THEREON. 